The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
The need to transfer heat away from one or more devices in order to keep temperatures from rising to destructive levels is common in many electrical and non-electrical applications. A common trait of such systems is that the power dissipating device is mated to the heat removal device at a common surface to surface interface. Minimal intervening thermal impedance is desired to minimize the temperature difference across the boundary.
Traditional power electronics applications utilize multiple heat sinks or a single common heat sink. These typically provide some form of fins in a gaseous or liquid cooling approach to effect heat transport away from the device.
To support the needs of industry, semiconductor manufacturers usually mount single or multiple devices on an insulating substrate (typically a ceramic substrate) in a flat geometry. In many power circuits, multiple devices are interconnected using combinations of flat direct-bonded-copper-to-ceramic islands and wires connected from die-to-die and to the substrate with a small links of wire. Usually such wire bonds are welded robotically. These interconnected objects are generally referred to as power modules.
Demand for smaller and lighter products with increased power density continues to drive engineered product development to achieve competitive advantages of smaller size and weight. As a result, the size and quantities of semiconductor dies on the substrate continue to increase to support higher currents. Simultaneously, the voltage levels are increased. Since power is proportional to the product of voltage and current, both dimensions of current and voltage are increased to produce higher power processing capability in the smallest volume possible. Progressively greater surface area is required in this flat approach. As the surface area increases so does the loop area of the circuitry. This loop area gives rise to parasitic inductance in the circuit which causes trapped energy to be produced that must be dissipated as heat every switching cycle. In this predominantly flat geometry a point of diminishing return may be reached where power loss and current density coupled with high voltage electric field limits make further increases in power density by volume problematic. In many circuits of interest, such as motor drives and power supplies, switching at high frequency is important to reduce the size of transformers and other magnetic devices. Such parasitic inductance therefore limits the ability to remain efficient and to allow further reductions of size.
Parasitic inductance is highly undesirable because it gives rise to voltage overshoot. Voltage overshoot must then be controlled by other means such as snubbers, which dissipate the trapped energy caused by the parasitic inductance. This results in power loss that impairs the overall efficiency of the electronics power conversion process and exacerbates the need for more cooling. Since losses due to parasitic inductance increase with switching frequency, the practical limit of maximum switching frequency may ultimately be linked to the loop area and geometry.
Those skilled in the art of high frequency switch mode power electronics circuit design will recognize that the goal of minimizing loop inductance amounts to reducing the geometry of the power circuit to the smallest area and ultimately the smallest volume possible. However, it should also be recognized that achieving these goals cannot be accomplished without also managing other constraints such as a) assuring that conductor size is adequate (to keep conductors from overheating); and b) assuring that clearances are adequate (to keep high voltage electric fields from breaking down the insulation or the surrounding gaseous atmosphere and causing an arc). A further emerging challenge of operating at high power levels are safety concerns if and when there is a catastrophic failure and an arc is created.
Additional practical considerations must also be considered, such as managing the cost of the solution so that economic feasibility is maintained in the solution and maintaining mechanical reliability of clamping forces over wide temperature variation etc.
Those skilled in the art will recognize that, in most cases, the goal of switching at higher frequencies is driven by a desire to minimize the size and weight of filters which are used to achieve power quality or to achieve necessary regulatory electromagnetic interference performance levels, or both. These filters may include inductors, capacitors, and transformers whose weight is to a first order inversely proportional to the switching frequency of the power converter, and which also require effective heat removal.
In summary, the trend towards design of power conversion systems is to make them smaller and lighter and handle more power. Stated differently, the ultimate goal in designing power conversion systems is to provide the highest power density by volume and/or weight possible.
Those skilled in the art will recognize that as power density is increased the power loss in any switch mode system must be carefully managed. One way to do this is to minimize loop inductance. In order to do this, as the circuit gets smaller, the needs of achieving high reliability voltage insulation, high thermal performance, and low conduction and switching losses become more difficult. Having to manage all of these constraints simultaneously challenges the limits of available technology. These simultaneous goals must be balanced altogether both technically and economically to achieve success in the business of power electronics.